1. Field of the Invention
The present invention relates to a mobile station, a mobile communication system, and a cell selection method.
2. Related Background Art
Conventionally, when a mobile station selects a cell as a reselection target, the mobile station measures a received level of a signal from a base station currently under communication therewith or a base station of a cell where the mobile station is camped, and received levels from base stations of neighboring cells. The mobile station compares the received levels and selects as a target cell a cell with the maximum received level from its base station, based on the result of the comparison (e.g., reference is made to Patent Document 1 and Patent Document 2).
The conventional cell selection method will be briefly described below with reference to FIG. 1. As shown in FIG. 1, mobile station 10 is camped on a cell C0 established by base station B0, and cells C1, C2, and C3 exist as cells neighbor to the cell C0. Mobile station 10 receives broadcast information M0 from base station B0 to acknowledge the existence of neighboring cells C1, C2, and C3. The broadcast information M0 is transmitted and received through a common broadcast channel or through a control channel individually dedicated to mobile station 10.
Mobile station 10 measures a received level L0 from base station B0, and received levels L1, L2, and L3 from the respective base stations B1, B2, and B3 forming the neighboring cells C1, C2, and C3. Thereafter, mobile station 10 compares the received level L0 with those L1, L2, and L3 and determines the propriety of a cell reselection. The comparison of the received levels is made, for example, using the following conditions (1) and (2).max(Li)=max(L1, L2, L3)  (1)max(Li)>L0+ΔL  (2)
Here max(argument 1, argument 2, . . . ) represents a function that gives a maximum argument among argument 1, argument 2, . . . Furthermore, L0 indicates the received level of the serving cell and ΔL a reselection hysteresis value.
When condition (2) becomes true, mobile station 10 selects a neighboring cell corresponding to max(Li) as a target cell and implements a reselection of the cell. On the other hand, when condition (2) does not become true, mobile station 10 implements no cell reselection.
[Patent Document 1] Japanese Patent No. 3233854
[Patent Document 2] Japanese Patent No. 3315869 (p2 and FIG. 4)